Method for using electrocoagulation in hydraulic fracturing

ABSTRACT

A method of improving natural gas release from a well via an enhanced hydraulic fracturing operation. The method includes capturing or retrieving the flow back from the well following the fracturing operation. The flow back or other source water is introduced to an electrocoagulation (“EC”) treatment process. EC treatment separates the water from other fracturing fluid components in the flow back and also removes bacteria and other contaminants. Thereafter, the EC-treated fluid is recycled for subsequent fracturing operations. The process may also be used to treat all source water, including fresh water delivered to the well before it is used as a fracturing fluid.

TECHNICAL FIELD

The invention disclosed here generally relates to hydraulic fracturingmethods for enhancing the production of a natural gas well. Morespecifically, the invention is directed to a method of enhancing thefracturing and natural gas release process by pre-treating water used inthe fracturing fluid and/or recycling treated flow back fluid or sourcewater previously used in the hydraulic fracturing process.

BACKGROUND OF THE INVENTION

“Hydraulic fracturing” is a common and well-known enhancement method forstimulating the production of natural gas. The process involvesinjecting fluid down a well bore at high pressure. The fracturing fluidis typically a mixture of water and proppant (the term “proppant”includes sand and synthetics). Other chemicals are often added to theproppant to aid in proppant transport, friction reduction, wettability,pH control and bacterial control.

Varying amounts of water are required in a typical hydraulic fracturingoperation. Water is usually trucked to the well head site from otherlocations, typically in large quantities. The water may come from avariety of sources that include untreated water from rivers, lakes, orwater wells. Once delivered to the well head site, the water is mixedwith the proppant particulates and then pumped down the well bore.

During the fracturing process, the fracturing fluid penetrates producingformations (sometimes called “subterranean formations”) at sufficienthydraulic pressure to create (or enhance) underground cracks orfractures—with the proppant particulates supporting the fracture for“flow back.” Sometimes the process is repeated a multiple number oftimes at the well site. When this is done, the well head is closedbetween stages to maintain water pressure of the fracturing fluid for aperiod of time.

The process creates a significant amount of fluid “flow back” from theproducing formation. Untreated flow back often is not recyclable insubsequent fracturing operations because of the contaminants itcontains. Flow back is normally hauled away and treated off-siterelative to the geographic location of the well head.

Hydraulic fracturing is very important to companies involved in theproduction of natural gas. These companies have made large investmentsin looking for ways to improve upon all phases of the fracturingoperation. One obvious drawback to fracturing involves the high cost ofhauling water to the well head site followed by retrieving and haulingaway the flow back by-product for off-site treatment and subsequentdisposal.

There have been many attempts at improving gas production that resultsfrom fracturing operations by varying the make-up and use of thefracturing fluid. Attempts at stimulating natural gas production viafracturing generally falls in two categories: hydraulic fracturing and“matrix” treatments.

Fracturing treatments stimulate gas production by creating more flowpaths or pathways for natural gas to travel up the well bore forretrieval. Matrix treatments are different in that they are intended torestore natural permeability of the underground formation followingdamage. The make-up of the fracturing fluid is often designed to addressdifferent situations of this kind by making adjustments in the materialand chemical content of the fluid and proppant particulates.

The methods and processes disclosed here involve the quality of thewater used to make up the fracturing fluid and treatment of flow backand other water-based fluids produced from hydraulic fracturing or othersource waters for gas retrieval operations. There are many advantages tothe methods disclosed here: First, the disclosed methods provide a meansfor significantly reducing trucking costs to and from the well head sitethat directly relate to the large quantities of water typically neededfor hydraulic fracturing. Second, the disclosed methods offer a viableway to recycle the water used as the fracturing fluid in an energyefficient treatment process at the well head site. Third, because of thenature of the treatment process, for reasons explained below, thedelivered or recycled water component in the fracturing fluid improvesflow back and increases the quantity of natural gas produced thatresults from the fracturing operation.

In sum, the methods and processes disclosed below serve to improvenatural gas production at a lower water treatment cost.

SUMMARY OF THE INVENTION

The invention disclosed here involves methods and processes forimproving natural gas release from a well following a hydraulicfracturing operation. The method involves first introducing a hydraulicfracturing fluid into a producing subterranean formation viaconventional means. The typical hydraulic fracturing fluid includes amixture of water and other proppant particulates (or fracturingcomponents). After the pressure on the fluid is released, at least aportion of the hydraulic fracturing fluid is captured from thesubterranean formation (preferably, as much as possible). As indicatedabove, this is typically referred to as “flow back.”

The captured fluid or flow back is separated from residual proppantparticulates and then introduced to an electrocoagulation (“EC”)treatment process. The EC treatment separates the water in the flow backfrom much of the inherent subterranean contaminants as well as otherfracturing fluid components. Thereafter, the treated water is clean ofcontaminants and may be recycled into the fracturing fluid that is usedin subsequent fracturing operations.

The EC treatment serves to reduce the viscosity of the fracturing fluid,which makes it function better in the underground or producingformation. Part of the viscosity improvement obtained via the ECtreatment process relates to bacterial content removal and reduction inturbidity, in addition to removal of other particulates.

It is conceivable that the same type of EC treatment can be used totreat fresh water delivered to the well head from off-site locations.Even though it is relatively clean, newly delivered fresh water maystill contain bacterial or other contaminants that impede the fracturingprocess. Therefore, EC treatment of water newly delivered to the wellhead site may be beneficial before it is mixed with proppantparticulates and used to initiate a fracturing operation.

The EC system uses the combination of a variable power supply, step-downtransformer(s), and an AC to DC rectifier to produce the requiredtreatment conditions (proper electric current level). The design reducesthe overall power consumed by EC cells in order to achieve clarity inthe treated water over a wide range of water conductivity.

The variable power supply outputs an alternating current (“AC”)typically in the range of 0 to 480 volts AC (“VAC”). The precise levelis determined or controlled by a programmable logic controller (“PLC”)that sets the VAC output. The VAC output from the power supply is thendelivered to the variable step-down transformer, which has a series of“taps” that further adjust the AC output prior to delivery to therectifier. The taps are adjusted upwardly or downwardly depending onwhether or not the desired operating current (or targeted current) isreceived by the EC cells within the system. The adjustment is made bymonitoring the ratio of AC current to DC current.

Based on results to date, the methods and processes disclosed here willsignificantly reduce conventional transportation and disposal costsattributable to water hauling and treatment in hydraulic fracturingoperations. Moreover, the desired water quality is achieved at loweredelectrical cost relative to known EC systems. Finally, use of themethods and processes disclosed here appear to generate better flow backreturn from the well, and increased natural gas production, because ECtreatment at the well head site decreases the volume of particles in thefluid that would otherwise be trapped in the fracture. EC treatment atthe well head site also helps to reduce the ability of the water to formscales and precipitants while reacting with formation and other metalsand minerals in the fracturing water. Not only does it immediatelyenhance production but it also improves the production life of the well.EC treatment provides other potential benefits such as overall reductionin proppant/chemical use and minimizing environmental impact because ofbetter point-source control of contaminated water.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference numerals refer to like parts throughoutthe various views, unless indicated otherwise, and wherein:

FIG. 1 is a schematic view of a well head site and illustrates thegeneral treatment and recycling of fluid (“flow back”) from the wellhead;

FIG. 2 is a schematic that is to be taken with FIGS. 3 and 4 and shows apre-treatment storage tank for holding the flow back captured from thehydraulic fracturing fluid process prior to EC treatment;

FIG. 3 is a schematic of a series of parallel EC treatment cells thatreceive fluid from the pre-treatment tank shown FIG. 2;

FIG. 4 is to be taken with FIGS. 2 and 3 and is a schematic showing aplurality of settling or “flocculation” tanks that receive fluidprocessed by the EC cells in FIG. 3, with the fluid being passed ontofinal stage processing through media filters;

FIG. 5 is a block schematic diagram showing the operational control ofthe EC system;

FIG. 6 is a block schematic diagram that illustrates electric currentcontrol for the EC system;

FIG. 7 is related to FIG. 6 and is a block diagram illustrating controlof the tap settings in a transformer that makes up a portion of the ECsystem; and

FIG. 8 is similar to FIG. 1, but illustrates treatment of waterdelivered to the well head site before its initial use in a fracturingoperation.

DETAILED DESCRIPTION

Referring first to FIG. 1, the general process will now be described.The process centers around the use of a portable electrocoagulation(“EC”) system 10 (further described below) that is brought to a naturalgas well head site 12. The EC system 10 is small enough to rest on atruck trailer bed (not shown in the drawings).

Referring to FIG. 8, as an example, water that is to be used in thehydraulic fracturing operation is delivered to the well head site, asschematically indicated at 14 (by truck or other means). Newly deliveredwater (reference 13) is processed by the EC system 10 and then mixedwith proppant particulates. It is then pumped (as illustrated at 16)down the bore at the well head location, thus introducing a hydraulicfracturing fluid into a subterranean formation (indicated at 17). Thisbasic fracturing process is well-known in the gas industry, with theexception of using EC technology. Likewise, many different variations onthe make-up and delivery of fracturing fluids and proppants have beenused in the industry such as, for example, the materials described inU.S. Pat. No. 7,621,330 issued to Halliburton Energy Services, Inc.(“Halliburton”).

As a person familiar with hydraulic fracturing operations would know,when the fracturing process is deemed to be completed, pressure isreleased at the well head 12, thus resulting in flow back of thefracturing fluid back up through the well head 12. Referring again toFIG. 1, the hydraulic fracturing fluid that makes up the flow back iscaptured, (indicated at 18) and passed directly to the EC system 10.Natural gas is retrieved (indicated at 15) and piped to a storagefacility (indicated at 19).

The EC system 10, which will be further described in greater detailbelow, uses an EC treatment process to separate the water from othercomponents in the flow back. The EC-treated water is then held in astorage tank 20. Thereafter, it is mixed with new proppant particulatesand recycled (indicated at 22) for subsequent hydraulic fracturingoperations.

For reasons described later, the EC system 10 will significantly reduceflow back parameters like turbidity and bacteria to very low levels.With the exception of sodium and chloride contaminants, other chemicalsin the flow back are likewise reduced via the EC treatment process.

Moreover, recycling the EC-treated water by subsequent mixing withconventional proppant particulates is beneficial to the hydraulicfracturing or fracking process. Processing the flow back (or deliveredfresh water) via the EC process 10 and recycling it in subsequentoperations positively affects viscosity of the fracking fluid (byreducing viscosity) and, consequently, affects the release of naturalgas from the subterranean formation.

The EC process reduces viscosity (μ) in Darcy's general equation:

$Q = {\frac{{- \kappa}\; A}{\mu}\frac{\left( {P_{b} - P_{a}} \right)}{L}}$

The reduction in μ is particularly acute with respect to diminishingimbibition in rocks less than 1 milli-Darcy. By reducing μ and,consequently, imbibition, the fractured interface is significantly lessdamaged, which benefits the recovery of the fracturing fluid (i.e., theflow back) and improves gas recovery from the well head.

The total discharge, Q (units of volume per time, e.g., m³/s) is equalto the product of the permeability (κ units of area, e.g. m²) of themedium, the cross-sectional area (A) to flow, and the pressure drop(Pb−Pa), all divided by the dynamic viscosity μ (in SI units, e.g.,kg/(m·s) or Pa·s), and the physical length L of the pressure drop.

The negative sign in Darcy's general equation is needed because fluidsflow from high pressure to low pressure. If the change in pressure isnegative (e.g., in the X-direction) then the flow will be positive (inthe X-direction). Dividing both sides of the above equation by the areaand using more general notation leads to:

$q = {\frac{- \kappa}{\mu}{\nabla P}}$where q is the filtration velocity or Darcy flux (discharge per unitarea, with units of length per time, m/s) and ∇P is the pressuregradient vector. This value of the filtration velocity (Darcy flux) isnot the velocity which the water traveling through the pores isexperiencing.

The pore (interstitial) velocity (v) is related to the Darcy flux (q) bythe porosity (φ). The flux is divided by porosity to account for thefact that only a fraction of the total formation volume is available forflow. The pore velocity would be the velocity a conservative tracerwould experience if carried by the fluid through the formation.

Water treated by EC is likely to provide better flow rates undergroundin pressure-driven fracturing operations according to the followingversion of Darcy's law (relating to osmosis):

$J = \frac{{\Delta\; P} - {\Delta\;\Pi}}{\mu\left( {R_{j} + R_{m}} \right)}$

where,

-   -   J is the volumetric flux (m·s⁻¹),    -   ΔP is the hydraulic pressure difference between the feed and        permeate sides of the membrane (Pa),    -   ΔΠ is the osmotic pressure difference between the feed and        permeate sides of the membrane (Pa),    -   μ is the dynamic viscosity (Pa·s),    -   R_(f) is the fouling resistance (m⁻¹), and    -   R_(m) is the membrane resistance (m⁻¹).

In both the general and osmotic equations, increased discharge orvolumetric flow is proportionate to decreased viscosity. Therefore, anytreatment method that is likely to reduce viscosity in a fracturingfluid is also likely to improve the outcome of the fracturing process interms of improvements to natural gas production.

Once again, water that is delivered to the fracturing or well head sitemay come from a variety of sources. Using river water, as an example,the water may be relatively clean but it will still contain varyingamounts of contaminants. Therefore, it may be desirable to use the ECsystem 10 for a threshold treatment of the water as it is delivered(thus reducing viscosity) and before mixing with sand or chemicals. Asindicated above, the EC system 10 is otherwise self-contained so that itis easy to move to and from the well head 12. FIGS. 2 and 3 illustratethe basic operating parameters of the system 10.

In the recycling scenario, the flow back 18 is delivered to apretreatment holding tank 24 (see FIG. 2). From there, the flow back ispassed to a manifold feed system 28 (see FIG. 3) via line 26. Themanifold system 28 distributes the flow back to a series of parallel ECtreatment cells, indicated generally at 30. Each EC treatment cell hasan internal configuration of charged plates that come into contact withthe flow back.

EC treatment cells with charged plate configurations have been ingeneral use with EC systems for a long time. However, to the extentpossible, it is desirable to select plate and flow-throughconfigurations that create turbulent flow within each cell. It isundesirable to generate significant amounts of flocculation within thecells 30 themselves. After treatment by the cells 30, the flow back isreturned to a series of settling tanks 32 (see FIG. 4) via line 34.

The EC treatment in the cells causes flocculent to be subsequentlygenerated in the settling tanks 32. There, the contaminants are removedfrom the water via a settling out process. Solid materials are removedfrom the settling tanks 32 and trucked off-site for later disposal in aconventional manner. The clarified water is then passed through sandmedia 36 (usually sand or crushed glass). Thereafter, the EC-treatedwater is passed onto the storage tank 20 (FIG. 1) for recycling insubsequent fracturing operations (see line 21 in FIGS. 1 and 4,respectively). Once again, the EC treatment positively improves theviscosity of the fluid (by reducing viscosity). Various pumps 37 areused at different points in the EC process to move the flow from onestage to the next.

There will be some variables in the overall EC treatment process fromone site to the next because of chemical and similar differences in thefracturing fluid or flow back. Similarly, there may be variations thatare dependent on the content of delivered water in those situationswhere the EC treatment process is used initially to treat incoming waterbefore it is used in a fracturing operation.

FIG. 5 is a schematic that illustrates the control logic for the ECsystem 10 illustrated in FIGS. 1-3. The EC system 10 utilizes anadjustable power supply 44. Three-phase power is delivered to the powersupply 44 at 480 volts AC (“VAC”) (schematically indicated at 46 in FIG.4). The output of the power supply 44 (indicated generally at 48) is avariable that is adjusted from 0 to 480 VAC by a controller 50. Thepower supply output 48 is delivered to a variable step transformer 51that further step down the AC voltage from the power supply 40 beforedelivering it to a three-phase rectifier 52.

Both the power supply 44 and transformer 51 are conventional powersystem components when standing alone. The transformer 51 includes aseries of “taps,” which would be familiar to a person having knowledgeof transformer systems. The “taps” provide different set points forstepping down the voltage across the transformer according to the powercurrent level needed by the EC system 10.

The three-phase rectifier 52 converts the output (see 54) from thetransformer 51 to direct current (“DC”). The three-phase rectifier 52 isalso a conventional component, standing alone.

The transformer 51 evens out or prevents current “spikes” that aretypical to the way adjustable power supplies work. The EC system 10 isadjusted to operate at a target current that maximizes EC celloperation. Part of this process involves imparting a charge to the fluidbeing treated without instigating significant amounts of flocculation inindividual cells.

That is, the target current is conducted through the flow back (or otherfluid under treatment) in the EC treatment cells 30 via the chargedplates within the cells. The target current may be set manually by theEC system operator, depending on the water quality of the flow backafter EC treatment. Alternatively, it may be set automatically via analgorithm described below:I _(target) =I_(user)−((Turb_(out)−Turb_(goal))+(Turb_(in)−Turb_(cal)))×(1/Flow)

-   -   Where:    -   I_(target)=Current system will maintain and hold to provide        treatment    -   I_(user)=Current set point user has specified to provide the        gross level of treatment    -   Turb_(out)=Turbidity out of treatment train    -   Turb_(goal)=Desired turbidity out of the system    -   Turb_(in)=Turbidity of the water to be treated    -   Turb_(cal)=Turbidity value to which the system is baseline    -   Flow=Flow rate through the treatment cells

The controller 50 is a conventional programmable logic controller. Thebasic control of current to the treatment cells 30 will now be describedby referring to FIG. 6.

The controller 50 ramps up to the target current 56 as follows.Reference numeral 58 (in FIG. 5) reflects the controller's constantmonitoring of DC current (I_(DC)) and AC current (I_(AC)) output fromthe transformer 51 and three-phase rectifier 52. The EC system 10 uses aproportional integral derivative algorithm (PID) to maintain cellcurrent to a set point defined by the user, as shown at 60. PIDs aregeneric algorithms that are well-known.

Unique to the present invention, the control logic includes a “powerquality” (“PQ”) calculation that is based on the following equation:

${PQ} = {\frac{I_{A\; C}}{I_{D\; C}} \times 100}$

Both the AC (I_(AC)) and DC (I_(DC)) current values are sensed followingrectification. The DC current (I_(DC)) is the averaged direct outputfrom the rectifier 52. The AC current (I_(AC)) is the residualalternating current from the rectifier 52. The DC and AC values reflectdifferent characteristics from the same wave form output by therectifier 52.

The tap settings in the transformer 51 are adjusted, as shown at 62,depending on the power quality (“PQ”) value. If the PQ is equal to orgreater than 60 (as an example), or alternatively, if the sensed currentis less than the target current, then the controller 50 adjusts thetransformer tap settings (reference 64).

The control logic for the tap adjustment 64 is further illustrated inFIG. 6. Transformer taps are adjusted either upwardly or downwardlydepending on the PQ calculation (referenced at 66). If PQ is equal to orgreater than 60, for example, then the controller shuts down the powersupply 68 (see, also, reference 44 in FIG. 4) for a brief period. Atthat point in time, the transformer taps are adjusted downwardly (item70). As a skilled person would know, if the transformers have a set offive taps, then they are selected one at a time in the direction thatsteps voltage down another step (with the process repeated iterativelyuntil the desired result is achieved. If PQ is not equal to or greaterthan 60, then the power supply is similarly shut down (see item 72), butthe transformer taps are instead adjusted upwardly (reference 74).

Returning to FIG. 6, if the current set point is not outside the rangespecified in control logic block 62 (that is, the current setting isacceptable), then the controller 50 checks the polarity timing function76. In preferred form, the EC system 10 is set to maintain polarityacross a set of plates inside the EC treatment cells 30 for a specifiedperiod of time. The control logic will loop through the sequence justdescribed (item 78) until the next polarity time-out is reached. At thatpoint in time, the controller 50 once again shuts down the power supply(see item 80) and switches the polarity 82 of the plates inside thetreatment cells to run until the next time-out period.

Referring again to FIG. 5, the controller 50 may also monitor incomingand outgoing flow rate (86) pH (88, 89), turbidity (90, 91), and otherfactors relating to the flow back via conventional sensor control logic84. The pH of the flow back may need to be adjusted upstream of the ECcells so that no flocculation occurs in the flow back before it reachesand passes through the treatment cells 30. Flow rates and pH andturbidity factors 86, 88, 89, 90, 91 may be continually andautomatically monitored by the controller 50. Depending on the qualityof the output from the settling tanks 32, and after filtering (see 36,FIG. 4), the treated flow back could be recirculated through the system(not shown) until the EC system's operation is stabilized. Otherwise,the treatment water is discharged (reference 94) to the water tank 20for recycling in the next hydraulic fracturing operation. Once again,the same basic treatment process is used if delivered water is treatedprior to any use as a fracturing fluid.

The use of EC technology to enhance hydraulic fracturing in natural gasapplications offers many advantages. The benefits of reduced viscositywere previously described. In addition, EC creates significant bacterialkill in the treated water—whereas bacteria in fracturing fluid isotherwise known to be undesirable. The direct field current generated inthe EC cells 30 serves to kill bacteria. If aluminum plates are used inthe cells 30, they will also generate aluminum hydrate which alsoaffects certain bacterial types.

In preferred form, stable operation of the EC system 10 involves no orminimum chemical adjustment to the flow, with the treatment relying onthe cell plate charge delivered by current control. It is preferred todeliver target currents in the range of 100 to 140 amps DC. These highcurrents can be achieved because of proper impedance matching providedby the variable step-down transformer 51 described above. It is alsomore power efficient to use a 3-phase rectifier (reference 52) in lieuof single-phase rectification. Different EC cell designs are possible.However, it is desirable to use cell designs that are capable ofdissipating the heat potentially generated by putting high current loadson the plates.

The foregoing description is not intended to limit the scope of thepatent right. Instead, it is to be understood that the scope of thepatent right is limited solely by the patent claim or claims thatfollow, the interpretation of which is to be made in accordance with theestablished doctrines of patent claim interpretation.

What is claimed is:
 1. A method for recycling fracturing fluid at a wellduring a hydraulic fracturing operation, comprising the steps of:introducing a hydraulic fracturing fluid into a subterranean formation,the hydraulic fracturing fluid including a mixture of water and otherfracturing components that include proppant particulates; capturing atleast a portion of the hydraulic fracturing fluid from the subterraneanformation after a fracturing operation, the captured hydraulicfracturing fluid having certain contaminants; introducing the capturedhydraulic fracturing fluid to an electrocoagulation (“EC”) treatmentprocess and using the EC treatment process to separate the water in thecaptured hydraulic fracturing fluid from other components in thecaptured hydraulic fracturing fluid; recycling the water separated viause of the EC treatment process to create a recycled hydraulicfracturing fluid, by directly mixing new proppant particulates intorecycled water that is separated via the EC treatment process, and usingthe EC treatment process to reduce the viscosity of the mixture ofrecycled water and new proppant particulates; and reintroducing therecycled hydraulic fracturing fluid into the subterranean formation fora subsequent hydraulic fracturing operation, wherein the EC treatmentprocess reduces the viscosity of the recycled hydraulic fracturing fluidduring the fracturing operation.
 2. The method of claim 1, wherein theEC treatment process includes: providing a variable power supply thatoutputs an alternating current (“AC”); rectifying the output of thevariable power supply from AC to direct current (“DC”); and varying theoutput of the variable power supply based on the ratio of AC to DCcurrent after rectification.
 3. The method of claim 1, further includingadjusting the pH of the captured hydraulic fracturing fluid during theEC treatment process to facilitate effluent flocculation.
 4. The methodof claim 1, including: using the EC treatment process to remove bacteriain the water separated from the captured hydraulic fracturing fluidbefore recycling the water in subsequent hydraulic fracturingoperations.
 5. A method for creating hydraulic fracturing fluid at awell during a hydraulic fracturing operation, comprising the steps of:creating a hydraulic fracturing fluid by mixing water and otherfracturing components that include proppant particulates; using anelectrocoagulation (“EC”) treatment process on the water prior to mixingproppant particulates directly into the water, and thereby using the ECtreatment process to reduce the viscosity of the mixture of water andproppant particulates when the mixture is used for the fracturingoperation; and introducing the hydraulic fracturing fluid into asubterranean formation for a hydraulic fracturing operation, wherein theEC treatment process reduces the viscosity of the hydraulic fracturingfluid during the fracturing operation.